The invention relates to a thin film transistor and, in particular, to a thin film transistor with a microlens.
Thin film transistors can act as image sensors capturing image or light signals in addition to driving devices on a liquid crystal display panel. Image sensors are an input interface and applications thereof include copiers, input for light pens or laser pointers, camera, palm or fingerprint recognition, interactive television, automatic light compensation, X-ray medical imaging or web-cams.
A pixel array of a conventional thin film transistor liquid crystal display (TFT-LCD) is shown in FIG. 1. Amorphous silicon thin film transistors (TFTs) in the pixel array are provided with a back channel etched structure (BCE structure), as shown in FIG. 2. Since amorphous silicon is a very good light absorbing material, especially in the range of visible light, the back channel etched thin film transistor is used as an image sensor for image capture. Fabrication of a TFT sensor, however, is constrained by fabrication process of an amorphous silicon thin film transistor display (α-Si TFT-LCD). Thus, the ratio of photocurrent to dark current of a TFT sensor is around or lower than 103. When applied as an image sensor, conventional TFTs present drawbacks as follows. When contrast ratio is low, images of full color or better quality cannot be captured, mistakes are often made when the conventional TFT is used in palm or fingerprint recognition, the TFT cannot normally function in all circumstances when used as an input for an interactive television or monitor, when the contrast ratio is low, it is very complicated to design a suitable backend sense amplifier, and, since photocurrent is smaller, the response time is longer.